Today one of my friends told me about a mother of one of her friends having an ongoing fever for 2 weeks. I immediately thought that this is likely not the result of an infectious disease, but of a malignancy of some sort.

What this person has is a fever of unknown origin (FUO). FUOs almost always require infectious disease consultation, represent a challenging aspect of infectious disease, and a problem I relish tackling.

Usually an FUO is the result of an infection, an autoimmune disease, or a malignancy. In those under the age of 65, infections make up the majority of cases. As one ages, however, infection becomes less likely as malignancy becomes more common.

A fever is best understood as a warning sign and adaptation from the immune system that can be caused by a myriad of different processes. Nevertheless, they prompt extensive work-ups in order to find the inciting cause, especially when present for two weeks.